The present invention relates to a semiconductor device suitably applicable to a thin film transistor (TFT), etc. for use in an image sensor or the like, in which mobility of carriers and yield are improved.
In a known image sensor in which a number of TFTs are arranged in a form of a matrix on a substrate, each of the TFTs is required to have excellent yield and high quality in order to improve quality of the image sensor.
FIG. 1 shows one example of such known TFT. The known TFT includes a substrate 31 made of glass or the like, a film 32 made of silicon nitride, a semiconducting layer 33 made of polycrystalline silicon, a source layer 34, a drain layer 35, a gate insulating film 36 made of silicon nitride, a source electrode 37, a drain electrode 38 and a gate electrode 39.
FIG. 2 shows schematically neighborhood of an interface between the semiconducting layer 33 and the gate insulating film 36 on a large scale. In this neighborhood, the gate insulating film 36 enters into recesses between adjacent crystal grain boundaries of polycrystalline silicon on an upper face of the semiconducting layer 33 so as to form insulating portions 40.
In the above known TFT, mobility of carriers is poor on the following ground. Namely, it is considered that since location of channels formed in the semiconducting layer 33 is restricted to an area disposed below the insulating portions 40, the semiconducting layer 33 cannot be used to full extent. Meanwhile, poor reproducibility and low yield of the known TFT will be attributed to the fact that since size and depth of crystal grain boundaries of polycrystalline silicon are nonuniform, size and depth of the insulating portions 40 formed between the crystal grain boundaries are also become nonuniform.
One example of such known TFT was introduced in a paper entitled "Poly-Si TFT fabricated by a low-temperature process utilizing XeCl excimer laser annealing" by Samejima et al. in a bulletin on lectures of the 46th meeting of the Japan Society of Applied Physics held in 1985.